For years hikers have always been told you can’t drink too much water — but now the New York Times reports that the biggest threat to marathoners is drinking too much water.
The Times article notes:

in their zeal to avoid becoming dehydrated, runners may end up drinking so much that they dilute their blood. Water rushes into cells, including cells of the brain. The swollen brain cells press against the skull, and the result can be fatal. The resulting condition is known as hyponatremia - too much water
“There are no reported cases of dehydration causing death in the history of world running,” Maharam said. “But there are plenty of cases of people dying of hyponatremia.”

Marathoners are being told to drink 8 fluid ounces for every 20 minutes.